1. Field of the Invention
The invention relates generally to a method and a reflector apparatus for concentrating solar energy, and more specifically relates to a solar concentration method and reflector for concentrating solar flux evenly on a solar cell array.
2. Description of the Prior Art
Paralleling the efforts aimed at reducing the cost of electrical energy generated by terrestrial photovoltaic (PV) systems by reducing the cost of solar cells and increasing their efficiencies, is the development of the concept of solar concentration for PV applications. As an alternative to the deployment of large-area solar cells in flat, non-concentrated arrays, concentrating devices and systems promise to greatly decrease the amount of cell area required in converting sunlight intercepted in a given area. Considerably less solar cell area is required in a system that uses concentrated sunlight because the efficiency of such cells increases logarithmically with the level of solar irradiance, up to the point where the heating of the cells prevents further gains despite cooling efforts. Thus, a significant cost advantage may be obtainable since solar cells typically are of two orders of magnitude more expensive per unit area than the materials used for the solar concentrating component of a photovoltaic conversion system. It therefore becomes feasible to employ relatively expensive high-efficiency solar cells in a PV solar conversion system that promises to produce electricity at a cost much lower than electricity provided by a comparable flat array configuration.
In a concentrating photovoltaic system, the sunlight is concentrated to relatively moderate levels, for example from ten to several hundred "suns." It is of great importance that this level of illumination be uniformly distributed across the face of the solar cells for optimum conversion efficiency. In a typical solar conversion system the solar cells in the plane of illumination are connected in series to obtain sufficient output voltage, and cells connected in series are limited in current to that of the cell with the lowest current. This places a premium on illuminating each cell equally. It is further noted that for each individual cell, non-uniform illumination of that cell will cause a loss of cell efficiency. Thus, it is desirable to provide a uniform flux density over each of the individual solar cells of an array and also over the entire array for optimum conversion efficiency. Regarding flux intensity, it is expected that modern PV conversion systems that concentrate flux on high-efficiency cells will attain optimum performance at concentration levels in the moderate range of 50 to 200 suns. Investigators using relatively expensive high-efficiency silicon solar cells have found peak performance efficiencies to be achieved with concentrations in the range of 75 to 100 suns. This is disclosed in Rios, M. Jr., and Boes, E. C., 1982, "Photovoltaic Concentrator Technology", Progress in Solar Energy, The Renewable Challenge Vol. 5, Part 3 of 3, Review and Indices, 1982 Annual Meeting, American Solar Energy Society, Houston, Tex., Franta, G. E., et al., eds., pp. 1563-1576.
The prior art contains various approaches to solar concentration for photovoltaic applications; however, existing systems generally do not satisfy the uniform flux concentration requirements, and the need for improved solar concentrators remains. One approach to the problem has been to use a Fresnel refractor lens such as a flat lens or a roof lens for refracting solar energy onto a flat PV plane. Curved Fresnel lenses are also employed in concentrator systems with some degree of performance success. However, these conventional compression-molded acrylic Fresnel lenses are relatively costly, require costly module housings, and are susceptible to thermal and mechanical failure. Other drawbacks of the Fresnel lens concentrator concept include transmission losses through the lens. A secondary concentrator has been proposed as a way of achieving higher concentrations (200 to 500 suns) and increasing the uniformity of the flux profile. This is discussed in the publication of Winston, R., and O'Gallagher, J., 1988, "Performance of a Two-Stage 500X Nonimaging Concentrator Designed for New High Efficiency, High Concentration Photovoltaic Cells, " Solar '88 Coleman, M. J., ed., Proceedings of the 1988 Annual Meeting, American Solar Energy Society, Inc., Cambridge, Mass., pp. 393-395. It is noted however, that for applications which do not require such high levels of concentration, the addition of a secondary concentrator adds cost and complexity.
Another major approach has been to concentrate sunlight by way of reflection, and the prior art provides several examples of the use of reflector systems for concentrating solar flux. Parabolic dish reflectors that are disclosed relate primarily to the solar thermal dish technology where highly localized and concentrated flux, of the order of thousands of suns, is required. Concentrators employing reflective elements that are concave spherical in shape have also been devised. See Authier, B. and Hill, L., 1980, "High Concentration Solar Collector of the Stepped Spherical Type: Optical Design Characteristics," Applied Optics, Vol 19, No. 20, pp. 3554-3561. These are also thermal, point-focusing designs that are not adapted for harnessing concentrated flux at the relatively moderate concentrations (75to 200 suns) found desirable for PV cell applications and do not provide a means of achieving such flux with uniformity over a target plane.
One reflector concentrator system that is specifically designed for PV applications is disclosed in the article, Kurzweg, U. H., 1980, "Characteristics of Axicon Concentrators for Use in Photovoltaic Energy Conversion", Solar Energy, Vol. 24, pp. 411-412. In this article the solution to the uniform flux concentration requirement is to use axisymmetric reflector-absorber combinations for yielding uniform flux density at the absorber surface. These axisymmetric concentrators involve non-flat target geometries such as the surface of an inner cone that is coated with solar cells. This contributes little to the advancement of a concentrator system that can obtain optimal performance by virtue of projecting uniform flux on a flat planar target containing an array of solar cells.
Yet another photovoltaic dish solar-electric concept is presented in Swanson, R. M., July 1988, "Photovoltaic Dish Solar-Electric Generator", Proceedings of the Joint Crystalline Cell Research, and Concentrating Collector Projects Review SAND88-0522, Sandia National Laboratories, Albuquerque, NM, pp. 109-119. A reflective parabolic dish is employed to focus sunlight at the entrance of a receiver cavity and to convert the non-uniform entering flux into a uniform flux at the plane of a solar cell array mounted in the receiver. Light entering the cavity diverges as it proceeds beyond the primary focus and eventually is reflected off the sides of the cavity for dispersal over the cell array plane. Unfortunately, the cost and complexity of such designs are increased by the requirement of a receiver.
In an example of prior art that is not specific to PV applications, U.S. Pat. No. 4,195,913 shows an optical integrator for electromagnetic radiation that has a plurality of reflective segments attached to a spherical concave surface. This attempt to produce a light beam of uniform intensity requires the individual attachment of many reflective surface elements and is rather complex. Thus it does not lend itself to low-cost fabrication methods and techniques required by the solar conversion industry in its drive to develop relatively inexpensive yet highly effective concentrators for PV conversion systems.
A novel and unique solar concentrator approach is provided by the present invention, which presents a solution particularly well adapted for optimal photovoltaic conversion, by projecting in a manner heretofore unavailable, moderately concentrated solar flux with uniform intensity across a solar cell target plane. The attributes of the present invention are reflected in the following objects.